Transcript GAses - Mr. Fischer

GASES
Gas Properties
 Four properties determine the physical
behavior of any gas:
 Amount of gas
 Gas pressure
 Gas volume
 Gas temperature
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Gas pressure
 Gas molecules
exert a force on the
walls of their
container when
they collide with it
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Atmospheric
pressure
 Torricelli barometer
 In the closed tube, the liquid
falls until the pressure
exerted by the column of
liquid just balances the
pressure exerted by the
atmosphere.
 Patmosphere proportional to
height of liquid in tube
Standard atmospheric
pressure (1 atm) is
760 mm Hg
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Units for pressure
 In this course we usually convert to atm
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Let’s practice…
 Standard atmospheric pressure:
 1 atm = 760 mmHg
 Convert 625 mmHg into atm
 Convert 2.5 atm into mmHg
Gas pressure
 A manometer compares the pressure of a gas
in a container to the atmospheric pressure
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Mixtures of Gases
 Each gas contributes to
the total pressure
 The pressure caused by
each gas is the partial
pressure of that gas
 Ptotal = PA + PB
 Each gas occupies the
entire container volume,
at its own pressure (the
partial pressure of that
gas)
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Mixtures of
Gases
 When a gas is collected over water, it is always “wet”
(mixed with water vapor).
 Ptotal = Pbarometric = Pgas + Pwater vapor
 Example: If 35.5 mL of H2 are collected over water at 26 °C
and a barometric pressure of 755 mm Hg, what is the
pressure of the H2 gas? The water vapor pressure at 26 °C
is 25.2 mm Hg.
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Relationships between gas properties:
pressure, volume, and temp
 1660 Robert Boyle investigates P and V:
 Indirect Relationship:
 Pressure Increases, Volume Decrease
 Pressure Decreases, Volume Increases
 PV = constant or P1V1 = P2V2
Let’s Practice…
 A sample of gas occupies 10 L at .800 atm.
What will the volume be if the pressure
decreases to .750 atm?
 A sample of gas occupies 25 L at 1.5 atm.
What will the new pressure be, if the volume
increases to 30 L?
Gas Pressure & Temperature
 Gas pressure is proportional to gas
temperature:
• relationship between pressure
and temperature is always linear
• all gases reach P = 0 at same
temperature, –273.15 °C
Pressure (psi)
• this temperature is
ABSOLUTE ZERO
temperature (°C)
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
Let’s practice…
P
 constant
T
P1 P 2

T1 T 2
Gas Laws:
Charles
 In 1787, Jacques Charles discovered the same
relationship between gas volume and
• relationship between volume
temperature:
and temperature is always linear
• all gases reach V = 0 at same
temperature, –273.15 °C
volume (mL)
• this temperature is
ABSOLUTE ZERO
temperature (°C)
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Let’s Practice…
V
 constant
T
V1 V 2

T1 T 2
A temperature scale for gases:
the Kelvin scale
 1860 English physicist, William Thomson
(Lord Kelvin), suggests a relationship
between kinetic energy and temperature.
 A new temperature scale was invented that
has zero = absolute zero
 The new temperature scale was named the
Kelvin or absolute temperature scale
 K = °C + 273.15
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Let’s Practice…
 K = °C + 273.15
 Convert 98.6 °C into Kelvin
 Convert 125 K into °C
Temperature and Kinetic
Energy
 The absolute (Kelvin) temperature of a
substance is directly proportional to the
kinetic energy of its molecules.
 Kinetic energy is the energy an object has because
of its motion.
 KE = 1/2mv2
Temperature and Kinetic
Energy
 Light molecules will move faster
 Heavy molecules will move slower
 All molecules at the same temp. have the
same kinetic energy.
 As temp. changes the velocity (speed)
changes:
 Increasing temp = increasing velocity
 Decreasing temp = decreasing velocity
 At absolute zero = velocity of zero (motion stops)
Gas laws:
Avogadro
 Avogadro’s hypothesis is
 Equal volumes of gases at the same temperature and
pressure contain equal numbers of molecules
 In mathematical terms, the ratio of gas volume to moles is
constant, if pressure and temperature V
do not change
 constant
n

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Putting it all together:
Ideal Gas Equation
 Combining Boyle’s Law, Charles’ Laws, and
Avogadro’s Law give one equation that includes all
four gas variables:
PV
R
nT
or PV  nRT
 R is the ideal or universal gas constant
 R = 0.08206 atm L/mol K (most useful)
 If P is in units of mmHg, multiply by 760 mmHg then
use 62.4 mmHg L/mol K
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Using the Ideal Gas Equation
 Ideal gas equation may be expressed two ways:
 One set of conditions: ideal gas law

PV = nRT
 Two sets of conditions: general gas equation

P1V1 = R = P2V2
n1T1
n2T2
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Examples
 What is the volume occupied by 20.2 g NH3
gas at –25 °C and 752 mm Hg?
 How many moles of He gas are in a 5.00 L
tank at 10.5 atm pressure and 30.0 °C?
 A 1.00 mL sample of N2 gas at 36.2 °C and
2.14 atm is heated to 37.8 °C while the
pressure is changed to 1.02 atm. What
volume does the gas occupy at this
temperature and pressure?
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Standard Molar volume and
Stoichiometry
 Scientists have chosen a set of standard
conditions (standard temp. and pressure)
STP:
 1 atm or 760 mmHg
 0°C or 273 K
 Standard molar Volume (for any gas)
 1 mole = 22.4 L
 At STP, 22.4 L of any gas contains one mole of gas
molecules (6.02 x 1023 molecules)
Let’s Practice…
 Convert .5 moles of gas into L
 Convert 12 L into moles
A Model for Gas Behavior
 The gas laws describe what gases do, but they do not
explain why.
 The Kinetic Molecular Theory of Gases is the model
that explains gas behavior.
 KMT was developed by Maxwell and Boltzmann in the
mid-1800s
 KMT is based on the concept of an ideal or perfect gas
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Ideal gas
 Composed of tiny particles in constant, random, straight-line




motion
Gas molecules are point masses, so gas volume is just the empty
space between the molecules
Molecules collide with each other and with the walls of their
container
The molecules are completely independent of each other, with no
attractive or repulsive forces between them.
Individual molecules may gain or lose energy during collisions, but
the total energy of the gas sample depends only on the absolute
temperature.
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